1. Field of the Invention
This invention relates to a foreign material removing device for eliminating materials which do not contribute to the development of latent images in image forming apparatus such as copying machines, facsimile machines and printers. This invention also relates to a development apparatus comprising such a foreign material removing device.
2. Discussion of Background
In the above-mentioned conventional image formation apparatus, a residual toner remaining on a latent image bearing member such as a photoconductor for use in electrophotography, after toner image transfer to a recording paper, is removed by a cleaning unit. Generally the amount of such a residual toner corresponds to 5 to 30% of the total toner deposited on such a latent image bearing member.
It has been proposed to reuse the removed toner by transporting the toner back to a development apparatus, as described in Japanese Utility Laid-Open Patent Application 52-155044. The toner which was removed by the cleaning unit, however, contains paper dust, aggregated toner particles, and low-electric charged toner particles (hereinafter referred to as foreign materials). Such a foreign-materials-containing toner tends to be deposited on the background of images during the development process, causing the so-called toner deposition on the background. For this reason, recently such recovered toner is not generally used again.
On the other hand, a demand for using such recovered toner is in fact recently increasing from the view point of effective utilization of resources.
Under such circumstances, it is considered to be an effective method to remove the above-mentioned foreign materials from the toner recovered by the cleaning unit before it is transported back to the development apparatus. In this method, however, the polarity of such recovered toner to be transported back to the development apparatus is so unstable that a method of removing materials which are not sufficiently charged to a predetermined polarity from the normal toner as being foreign materials cannot be adopted in practice.
As a countermeasure to this problem, in particular in a development apparatus using a two-component developer consisting of a toner and a carrier, there can be considered a method of mixing the recovered toner with part of the two-component developer to be transported to the development apparatus before the recovered toner is transported to the development apparatus, thereby sufficiently charging the recovered toner to the desired polarity, and removing only the foreign materials from the sufficiently charged toner by use of electrostatic force.
This method, however, has the shortcomings that a large-scale unit is necessary for transporting the developer from the development apparatus and mixing the developer and the recovered toner, making the development apparatus large in size and costly. Furthermore, this method has the risk that the recovered toner cannot be necessarily sufficiently charged to the desired polarity by mere mixing the toner with the developer under transporation to the development apparatus, so that the foreign materials cannot be satisfactorily removed from the toner, and if the charging of the recovered toner is insufficient, the insufficiently charged toner may also be removed as being foreign material and high utilization of the recovered toner cannot be attained.
In addition to the above method, there has been proposed a method of transporting a toner recovered by a cleaning unit to a development apparatus and charging the toner to the desired polarity within the development apparatus to reuse the toner.
For example, Japanese Laid-Open Patent Application 59-232561 discloses the following method: A developer supplied to a development sleeve in a development unit. In the development unit, there is disposed a bucket roller portion with an built-in developer-stirring mechanism provided in the axial direction of the bucket roller portion for stirring the developer in the development unit. Furthermore, inside the developer-stirring mechanism, there are provided a low-charged toner separation mechanism, and a low-charged toner circulation portion through which only a toner which has been triboelectrically charged to a predetermined charge quantity is returned to the development unit to reuse the toner.
In the above development apparatus, the developer-stirring mechanism is provided within the bucket roller, and the low-charged toner separation mechanism, and the low-charged toner circulation portion are provided within the developer-stirring mechanism. Thus this apparatus is very complicated in the structure and therefore costly. Furthermore, the charging of the toner is not sufficiently carried out even by relatively strong triboelectric charging of the toner with the carrier on a doctor portion on a development sleeve. Despite this fact, there is no triboelectric charging mechanism capable of strongly triboelectrically charging the toner contained in the low-charged toner circulation portion. Therefore, there is the risk that the toner cannot be sufficiently charged even by the toner circulation in the low-charged toner circulation portion for an extended period of time. Moreover, since foreign materials such as paper dust and aggregated toner particles merely accumulate within the low-charged toner circulation portion, it is extremely difficult to eliminate such foreign materials from the development apparatus.
Japanese Laid-Open Patent Application 57-104956 describes that in order to prevent the toner deposition on the background of images which may be caused by a low-charged toner within a development apparatus, a member made of a material whose triboelectric series is the same as that of a photoconductor is disposed in a position before a development zone in such a manner as to be in contact with a developer held on a development roller, and a low-charged toner is triboelectrically charged by this member and caused to deposit the toner on the member, whereby the low-charged toner is removed from the developer held on the development roller.
The above-mentioned mechanism, however, cannot eliminate the foreign materials such as paper dust and aggregated toner from the developer and there is the risk that such foreign materials are bonded to the toner or adhere to the surface of the carrier during an extended image formation process, so that the charging performance of the toner is caused to deteriorate and the toner deposition on the background of images takes place.
Japanese Laid-Open Patent Application 60-256169 describes a foreign material removing device for removing foreign materials from a developer, comprising first magnetic field generating means for magnetically holding a developer and for generating an alternate magnetic field by which the developer can be sufficiently agitated, and second magnetic field generating means disposed in close vicinity with the first magnetic field generating means, in which the lines of magnetic force held by the first and second magnetic field generating means are caused to continuously cross in a space near the two magnetic field generating means, so that the developer present in the space is caused to fly successively in such a manner as to depend upon the magnetic force, and only the foreign materials contained in the developer, which are not caused to fly, are separated and eliminated therefrom.
In the above foreign material removing device, however, there is the risk that if the above-mentioned space is too large, not only the foreign materials but also part of the developer is separated and eliminated from the developer, while when the space is too small, the foreign materials cannot be sufficiently eliminated.
The Japanese Laid-Open Patent Application 60- 256169 further describes that the developer may be introduced from the development apparatus into this foreign material eliminating device. However, there is the risk that unless the position for a developer inlet of a developer transportation means for transporting the developer from the development apparatus to the foreign material eliminating device is appropriately set, a developer which contains an insufficiently triboelectrically charged toner is also transported into the foreign material removing device to eliminate the toner, which may be used as proper toner if properly charged, as being a foreign material. Thus, the recovered toner cannot be used effectively.